Core-crush resistant fabric and prepreg for fiber reinforced composite sandwich structures

ABSTRACT

A core crush resistant prepreg for use in making a fiber reinforced composite panel structure is provided. The prepreg comprises a woven fabric consisting essentially of carbon fiber tow strands impregnated with a hardenable polymeric resin composition. Typically the fabric has an areal weight of from about 180 to about 205 grams per square meter. The prepreg has an average fiber tow aspect ratio of less than about 15.4, a prepreg thickness of at least about 0.245 mm, and a prepreg openness of at least about 1.2 percent but less than about 6.0 percent. Preferably, the resin composition is predominantly viscous in nature and has a tan δ value of between 0.9 and 2.0 at an elevated temperature between 70° C. and 140° C., and an average epoxy functionality of greater than 2.0. A method for evaluating core crush resistance properties of a prepreg is also provided. The method includes determining a fiber tow average aspect ratio of the prepreg, determining a prepreg thickness, and comparing said average fiber tow aspect ratio and prepreg thickness to a set of predetermined values.

FIELD OF THE INVENTION

[0001] The invention relates to composite sandwich structures,preferably honeycomb core, composite sandwich structures, and to fabricand prepreg components for such composite structures. More particularly,the invention relates to core crush resistant, honeycomb core compositesandwich structures, particularly those composite structures used in theaerospace industry, and to fabrics and prepregs for making suchcomposite sandwich structures.

BACKGROUND OF THE INVENTION

[0002] Honeycomb core composite sandwich structures find widespread usein the aerospace industry as panel components in various aerospacestructures. The honeycomb core composites are formed from a lay-up ofprepreg skin plies encompassing a honeycomb core, the latter typicallyhaving beveled edges. The prepreg plies may be fabrics, tapes, ornon-wovens that have been pre-impregnated with a thermosetting,thermoplastic or other polymeric resin. The fabrics used to form theprepregs are woven fabrics, formed primarily or entirely of highmodulus, reinforcing fibers in the form of continuous filament tows.Curing of the lay-up is carried out in a high temperature, high pressureenvironment, typically in an autoclave.

[0003] The technical requirements of aerospace end uses generallydictate that the prepregs and prepreg components meet a rigid set ofchemical, physical, and mechanical specifications including overallprepreg basis weight, fiber modulus, and resin flow rate. The basisweight of the prepreg and the high strength properties of the fibers andthe resin, in combination with the strength properties of the honeycombcore component, impart high strength-to-weight, and highstiffness-to-weight ratios to the final composite structure. Inaddition, the flow rate characteristics of the resin and the highpressures used to cure the composite, minimize porosity, i.e., theinclusion of voids and through holes, that might impair strength, thedesired impervious nature, and/or surface smoothness of the finalhoneycomb panel sandwich structure.

[0004] Even though honeycomb core composite panels have long been usedin the aerospace industry, manufacture of these structures is stillplagued by high reject scrap levels, generating substantial quantitiesof unusable scrap and impacting negatively on manufacturing economics.Partial collapse of the honeycomb core during curing of the composite,known in the industry as “core crush”, is a particularly common reasonfor rejection of cured panels. Core crush is typically observed in thebeveled edge or chamfer region of the honeycomb structural part.

[0005] Substantial effort and research extending over many years havebeen directed to the core crush problem. For example, U.S. Pat. No.5,685,940 to Hopkins discloses an improved tiedown method to produce orprevent core crush and ply wrinkling in honeycomb sandwich structures. Ascrim-supported barrier film is placed between the fiber-reinforcedresin composite laminates and honeycomb core to prevent resin flow fromthe prepreg into the honeycomb core. A tiedown ply between the core andthe barrier film is used to reduce slippage of the barrier film relativeto the core during curing. In addition, a film adhesive having a curingtemperature lower than that of the laminate resin is placed between thetiedown plies just outside the net trim line. During the curing process,cured film adhesive bonds the tiedown plies to one another before thecuring of the prepreg laminates, thus strengthening the tiedown andreducing core crush. The Hopkins patent also discusses other methods andstructural modifications which have been proposed for minimizing oreliminating the core crush. Nevertheless, core crush remains asignificant problem in the industry.

SUMMARY OF THE INVENTION

[0006] The present invention provides a core crush resistant prepreg foruse in making a fiber reinforced composite sandwich structure. Use ofthe prepreg of this invention, can significantly reduce the degree ofcore crush as compared to conventional structures.

[0007] In accordance with a first aspect of the invention, it has beenfound that the core crush problem associated with honeycomb corecomposite sandwich structures can be significantly reduced bycontrolling construction of the fabric used to prepare the prepreg. Inparticular, it has been found that core crush can be substantiallyreduced by controlling the cross sectional aspect ratio of the carbonfiber tow in the prepreg, the average thickness of the prepreg and theopenness of the prepreg, as measured by visual inspection. Inparticular, prepregs according to the invention are made from fabricshaving an areal weight range of from about 150 to 400 grams per squaremeter. The prepreg has an average tow aspect ratio of less than about15.5, an average prepreg thickness of at least about 0.245 mm, and/or anopenness of at least about 1.2% but less than about 10.0%.

[0008] While not wishing to be bound by theory, it is believed thataverage tow aspect ratio, prepreg openness and prepreg thicknessdetermine the frictional force between prepreg plies during the curingstep in the manufacture of honeycomb core composite sandwich structures.When prepreg properties of tow aspect ratio, prepreg thickness andprepreg openness are maintained within the ranges set forth above,sufficient frictional force is provided between prepreg plies such thatthe innermost prepreg plies, adjacent the honeycomb core, are restrainedfrom slipping during the curing process to thereby eliminate or minimizecore crush.

[0009] It has also been found, according to the present invention, thatwhen the tow aspect ratio, prepreg thickness and prepreg openness areoptimized to minimize core crush, the porosity of the final honeycombcore composite sandwich structure can be unacceptable. The porosityproblems can be especially prevalent in thicker composite sandwichstructures and especially when a low flow resin system is used toimpregnate the prepreg. The present invention employs a hardenablepolymeric resin composition having a flow rate higher than the flow rateof resins traditionally used in commercial practice in the aerospaceindustry in prepregs for honeycomb core composite sandwich structures inorder to maintain acceptable porosity in the final composite structure.Therefore, prepregs according to the present invention are impregnatedwith a hardenable polymeric resin composition having rheology which ispredominately viscous in nature, such that the ratio of viscous toelastic components of the viscosity, i.e., tan δ, is within thefollowing defined ranges.

[0010] Prior to significant resin cross-linking or curing, the resincomposition used in this invention preferably has a tan δ of betweenabout 1.2 and about 2.0, preferably between about 1.5 and about 1.8,more preferably about 1.35, at 70° C.; or, a tan δ of between about 0.7and about 2.0, preferably between about 0.9 and about 1.8, morepreferably about 1.35, at 100° C.; or, a tan δ of between about 0.5 andabout 1.7, preferably between about 0.7 and about 1.5, more preferablyabout 1.35, at 140° C.

[0011] Preferably, the tan δ of the resin composition is from about 0.5to about 2.0, more preferably between about 1.0 and about 1.8, mostpreferably about 1.35, throughout the elevated temperature range of fromabout 70° C. to about 140° C., or if the minimum viscosity temperatureis below 140° C., the range of from about 70° C. to the minimumviscosity temperature.

[0012] More preferably, prior to significant resin cross-linking orcuring, the resin composition has a tan δ of between about 1.0 and about2.0, more preferably between about 1.2 and about 1.8 at about 70° C.;between about 0.7 and about 2.0, more preferably between about 1.0 andabout 1.7 at about 100° C.; and, between about 0.5 and about 2.0, morepreferably between about 0.6 and about 1.7 at about 140° C., or at theminimum viscosity temperature, if the minimum viscosity temperature isbelow 140° C.

[0013] Preferably, the resin composition comprises an epoxy resin andhas an average epoxy functionality of greater than 2.0.

[0014] According to another aspect of the invention, a prepreg forattaining good core crush performance while minimizing porosity inhoneycomb core composite sandwich structures is provided havingdifferent tow aspect ratios in the warp and weft directions. Inparticular, prior art prepregs have been prepared using identical carbonfiber tows as warp and weft components. In accordance with this aspectof the invention, it has been found that different carbon fiber towconstructions provide different tow cross-sectional aspect ratios. Byusing different tows to form the warp and weft components of the fabriccomponent of the prepreg, it is possible to optimize and balance thefrictional properties of the prepreg, thus minimizing the core crushproperties of the prepreg, while also minimizing any substantialporosity increase in the final composite structure. In accordance withthis aspect of the invention, the prepreg is formed from differentcarbon fiber tows in the warp and weft directions. The prepreg has a towaspect ratio of no greater than about 13.0 in one of the warp and weftdirections and a tow aspect ratio of at least about 13.5 in the other ofthe warp and weft directions. In addition, the prepreg has an opennessof no greater than about 5.0%. Preferably the prepreg has a prepregthickness of from about 0.250 mm to about 0.275 mm. Prepregs accordingto this aspect of the invention, i.e., fabrics having hybrid tow aspectratios, are well suited for use with a wide variety of resins systems,e.g. of varying rheologies.

[0015] In accordance with yet another embodiment of the presentinvention, there is provided a prepreg which comprises in theresin-impregnated fabric, tow strands having a predeterminedsubstantially stable, non-round cross sectional shape. Preferably, suchtow strands have an average fiber tow aspect ratio of from about 12.0 toabout 14.5, and a prepreg openness of no great than about 5.0%. It hasbeen discovered that such prepregs can substantially reduce both corecrush and porosity in fiber reinforced composite structures.

[0016] Thus, the prepregs of the present invention are capable ofsubstantially reducing core crush and porosity in honeycomb corecomposite structures. However, no complexities are added to the lay-upand/or curing process for preparing the panels. Moreover, the presentinvention does not introduce abnormal or unusual components into thecomposite structure. Thus, the present invention can providesubstantially reduced core crush and substantially reduced reject levelswithout requiring substantial modification to the conventionalprocedures and/or lay-up arrangement used to prepare honeycomb corecomposite sandwich structures. In addition, because various fiber towscan be manipulated in accordance with the present invention to impartdesirable prepreg properties so as to reduce core crush, this inventionallows prepreg manufacturers greater freedom in selecting fiber tows orfabrics for use in preparing prepregs, and thus less dependence onspecific specialty fiber tow manufacturers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates a woven fabric for use in the prepreg of thisinvention;

[0018]FIG. 2 illustrates a prepreg prepared from the woven fabric ofFIG. 1 by resin impregnation thereof;

[0019]FIG. 2A is a diagrams illustrating the cross section of theprepreg of FIG. 2 taken along the line 2A-2A thereof and illustrates thethickness of the prepreg and the aspect ratio of the tows along onedirection of the prepreg;

[0020]FIG. 2B illustrates the cross section of a prepreg taken along thecenter line of a tow strand along one direction of a prepregdemonstrating the thickness of the prepreg and the predeterminedsubstantially stable round cross sectional shape as well as the towaspect ratio of the tow strands along the other direction of theprepreg.

[0021]FIG. 2C illustrates the cross section of a prepreg taken along thecenter line of a tow strand along one direction of a prepregdemonstrating the thickness of the prepreg and the predeterminedsubstantially stable non-round cross sectional shape as well as the towaspect ratio of the tow strands along the other direction of theprepreg;

[0022]FIG. 3 illustrates a woven fabric for use in the prepreg of thisinvention in which the weft tows have a different tow construction andaspect ratio as compared to the warp tows;

[0023]FIG. 4 illustrates a prepreg made from the woven fabric of FIG. 3;

[0024]FIG. 4A illustrates the cross section of the prepreg of FIG. 4taken along the line 4A-4A and illustrates the tow aspect ratio in afirst direction;

[0025]FIG. 4B illustrates the cross section of the prepreg of FIG. 4taken along the line 4B-4B and illustrates the tow aspect ratio in theother direction;

[0026]FIG. 5 is a graph illustrating the correlation between prepregthickness and core crush degree in honeycomb core composite sandwichstructures;

[0027]FIG. 6 is a graph illustrating the correlation between prepregaverage fiber tow aspect ratio and core crush degree in honeycombcomposite sandwich structures;

[0028]FIG. 7 is a diagram of a standard core crush panel used fortesting the core crush properties of prepregs of this invention;

[0029]FIG. 7A is a cross sectional view of the core crush panel of FIG.7 taken along the line A-A and illustrates the lay-up of the prepregplies and the honeycomb core before curing; and

[0030]FIG. 8 is a diagram illustrating the determination of the corecrush area in a standard core crush panel.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0032] Illustrated in FIG. 1 is a woven fabric 10 that can be used toprepare the prepreg of this invention. Although other conventionalfabrics such as knitted fabric and nonwoven fabric can be used, a wovenfabric is preferred. Furthermore, a variety of tow orientations, e.g.,±45°, 0°/90°, etc., may be used to weave suitable fabrics for thepresent invention. Typically, a two-dimensional biaxially woven fabricis preferred. The woven fabric 10 has a plurality of warp tow strands 12interweaving with a plurality of weft tow strands 14. The term “tow” and“tow strand” are used herein interchangeably to refer to the yarn usedto form a woven fabric. As will be apparent from the description below,each tow or tow strand is essentially a bundle of a plurality of fiberfilaments. As used herein, “warp” and “weft” are used herein to refer tothe two different directions in which the tow strands are oriented in atwo-dimensional woven fabric. Although in the textile art, warp towstypically are referred to yarns that run parallel to the selvage orlonger dimension of a fabric, the term “warp” as used herein can meaneither of the two directions, and “weft” the other direction.

[0033] As illustrated in fabric 10, a plurality of openings 16 areformed by the interweaving warp tow strands 12 and weft tow strands 14.Typically, the openings 16 are square or rectangular openings, i.e., thewarps and wefts cross at a substantially right angle. Although otherfabric patterns such as basket weave, crow foot satin weave, leno weave,mock leno weave, or unidirectional weave can be used, satin weave, twillweave, and plain weave are most preferred for the prepreg of the presentinvention. As is well known in the art, in a plain weave pattern asillustrated in FIG. 1, each weft tow passes successively over and undereach single warp yarn in alternating rows. In a satin weave pattern,warp tow strands outnumber the weft tow strands, and are characterizedby floats that run in the warp direction on the face in such a manner asto reflect light, producing gloss, luster, or shine. Typically, eachweft tow floats over two or more warp tows. In twill weaves, tows areinterlaced and dominant diagonally lined patterns are created.Typically, a series of floats are staggered in a definite pattern in thewarp direction, either to the left (left-hand twill), to the right(right-hand twill), or equally to the left and right in a zigzag effect(broken twill), which produces no diagonal lines.

[0034] Each tow strand 12 and 14 is formed from a plurality ofcontinuous filaments as will be apparent. The term “filament” and“fiber” are used herein interchangeably to mean an individual fiber orfilament of the multi-fiber or multi-filament tow. Various differenthigh modulus reinforcing fibers can be used to form the tows such ascarbon fibers, fiberglass, or aramid. Preferably, carbon fibers areutilized. This is because generally they have desirable properties ofhigh tensile strength, high modulus, and low density and good resistanceto adverse environmental factors such as high temperature, high moistureand high acidity. As is known in the art, carbon fibers are generallymade by converting various precursor organic polymeric fibrous materialsto a carbonaceous form under high temperature while retaining theoriginal fibrous configuration essentially intact except that the fibercross sectional size is normally decreased and/or the fibercross-sectional shape can be changed because of stretching duringcarbonization processing. Generally, for purposes of this invention,carbonization can be conducted either before or after the precursorfilaments are bundled together to form a tow.

[0035] As used herein, “carbon” fibers or “carbonaceous” fibersgenerally refer to graphite fibers and amorphous carbon fibers. Graphitefibers consist essentially of carbon and have a predominate x-raydiffraction pattern characteristic of graphite. Amorphous carbon fibersordinarily refer to fibers consisting essentially of carbon andexhibiting an essentially amorphous x-ray diffraction pattern.

[0036] The carbon fibers typically used for the tow strands arestructural textile fibers and have a standard tensile modulus, orYoung's modulus of above about 10×10⁶ psi, e.g., from about 10×10⁶ psito about 120×10⁶ psi, preferably from about 20×10⁶ psi to about 100×10⁶psi, more preferably from about 25×10⁶ psi to about 75×10⁶ psi, and mostpreferably from about 30×10⁶ psi to about 45×10 psi. Suitable carbonfibers should have an individual denier of from about 0.2 to about 1.0g/9000 m, preferably from about 0.3 to about 0.8 g/9000 m, morepreferably from about 0.4 to about 0.7 g/9000 m, and most preferablyfrom about 0.55 to about 0.65 g/9000 m. The diameter of each singlecarbon fiber filament can be in the range of from about 0.5 to about 50μm, preferably from about 1 to about 25 μm, more preferably from about 5to about 15 μm, and most preferably about 10 μm.

[0037] Each carbon fiber tow strand can have a total filament count offrom about 1,000 to about 80,000, preferably from about 2,000 to about50,000, more preferably from about 3000 to about 18,000, and mostpreferably 3,000 to about 12,000. For example, a typical filament countof a fiber tow strand can be 3,000, 6,000, 12,000, from about 1,000 toless than about 3,000, or can be greater than 3,000 but no greater than18,000. Typically, the fabric used in the prepreg of this inventionshould have an areal weight of from about 150 to about 400 grams persquare meter, preferably from about 160 to about 250 grams per squaremeter, more preferably from about 180 to 205 grams per square meter, andmost preferably from about 185 to about 201 grams per square meter. Asapparent to a skilled artisan, a fabric with an areal weight withinthese ranges is particularly suitable in making fiber reinforcedcomposite structures for aviation end uses.

[0038] In order to make the prepreg of the present invention, the fabricas described above is impregnated with a polymeric matrix resincomposition. Generally any resin compositions used in the art forimpregnating prepregs used for making fiber reinforced compositestructures can be suitably used in the present invention. For example,such suitable resin compositions are disclosed in e.g. U.S. Pat. No.4,599,413 to Moulton et al., U.S. Pat. No. 5,626,916 to Kishi et al.,and European Patent Application EP 0819723 Al to Kishi et al., each ofwhich are incorporated herein by reference.

[0039] Typically, a suitable resin composition comprises from about 40%to about 95% by weight of polymeric matrix resin, from about 5% to about40% by weight of curing agent, and from about 0% to about 20% by weightof a flow control agent. Many different polymeric matrix resins known inthe art can be used in the present invention. Examples of suitablepolymeric matrix resin include, but are not limited to, epoxy resins,phenolic resins, polyester resins, polyimide resins, polybenzimidazoles,polyurethanes, vinyl ester resins, bismaleimide resins, polyetherimide,polyetherketone, polyamides, etc. Polymeric matrix resins are processedin a variety of ways such as hardening a liquid, melting, foaming, andsolvent activation. However, a thermosetting epoxy resin is mostpreferred, especially epoxy resins derived from amines, phenols or vinylcompounds. As is known in the art, epoxy resins utilize epoxide groupsas the functional groups in the curing reaction. Suitable thermosettingepoxy resins may include, but are not limited to,diglycidyl-p-aminophenol, triglycidyl aminocresol,triglycidyl-p-aminophenol, tetraglycidyl diaminodiphenylmethane,tetraglycidyl ethers of methylenedianiline, bisphenol A type epoxyresins such as diglycidyl ethers of bisphenol A, bisphenol F type epoxyresins such as diglycidyl ethers of bisphenol F, bisphenol S type epoxyresins, phenol novolac type epoxy resins, cresol novolac type epoxyresins, resorcinol type epoxy resins, epoxy resins with a naphthaleneskeleton, biphenyl type epoxy resins, dicyclopentadiene type epoxyresins and diphenylfluorene type epoxy resins, etc. These resins can beused individually or in any appropriate combinations. Preferred epoxyresins are those having an epoxy functionality of at least 2.0, morepreferably greater than 2, because of the technical demands imposed byaerospace end uses. In other words, bifunctional, trifunctional, orhigher functional epoxy resin are preferred. In particular,glycidylamine type epoxy resin and glycidyl ether type epoxy resinhaving a functionality of greater than 2 are preferred.

[0040] Curing agents are those compounds having an active group that canreact with a functional group in the polymeric resin, for example, anepoxy group of an epoxy resin. Suitable curing agents for epoxy resinsinclude, but are not limited to, diaminodiphenyl methane,diaminodiphenylsulfone, various substituted ureas, aminobenzoates,various acid anhydrides, various isomers of dicyandiamide, phenolnovolac resins, cresol novolac resins, polyphenol compounds, imidazolederivatives, alphatic amines, tetramethylguanidine, carboxylicanhydrides, carboxylic acid hydrazides, carboxylic acid amides,polymercaptan, Lewis acid complexes such as a boron trifluorideethylamine complex, and the various heterocyclic multifunctional amineadducts as disclosed in the aforesaid U.S. Pat. Nos. 4,427,802 and4,599,413. Any of the above curing agents can be used eitherindividually or in various combinations in the resin composition of thepresent invention.

[0041] As is well known in the art, flow control agents in a resincomposition are used to adjust the viscoelasticity of the resincomposition. The flow control agents used in the present invention arealso often utilized with resin compositions to modify the mechanicalperformance of the cured composite article, e.g., by providingtoughness. Suitable flow control agents may include, but are limited to,e.g., solid rubbers, liquid rubbers, thermoplastic resin elastomers,organic and inorganic particles, and short fibers. Certainthermoplastics that are soluble or partially soluble in epoxy resins mayalso be used as flow control agents. Such thermoplastics include, e.g.,polysulfones, polyether sulfones, polyether imides, and polyethyleneoxide. The various flow control agents can be used individually or incombinations.

[0042] Typically, both a liquid rubber or elastomer and a solid rubberor elastomer, which are fully or partially soluble in the epoxy resinsare included. As is known in the art, when solid rubbers are used in anepoxy resin composition, the temperature dependence of theviscoelasticity function of the resin composition is decreased and thesurface smoothness of the skin panel in the cured fiber reinforcedcomposite sandwich structure is increased. Preferably, the solid rubberand/or liquid rubber included in the resin composition includes one ormore functional groups such as carboxyl groups and amino groups, whichreact with the epoxy groups of an epoxy resin in the resin composition.Examples of preferred solid rubbers include, e.g., solidacrylonitrile-butadiene rubber, hydrogenated nitrile rubber, etc.Various other solid and liquid elastomers known in the art can all beused.

[0043] Particulate flow control agents that are included in the resincomposition may also function as fillers or extenders. Examples of suchan agents include, e.g., fumed silica, mica, calcium carbonate, calciumphosphate, glass, metal oxides, cellulose, starch, clays, diatomaceousearth, etc.

[0044] Other materials such as catalysts, antioxidants, chain extenders,reactive diluents, and the like all of which are known in the art, canalso be optionally included. Although any resin composition as describedabove can be useful in the present invention, it is preferred however toemploy an epoxy resin composition which exhibits significant viscousflow that is higher than that of some of the conventional resins used inprior art prepregs used for preparing composite structures for aviationuses. In particular, it is preferred to use an epoxy resin compositionwherein the elastic and viscous components of the composition are ofsimilar magnitudes. For example, the preferred epoxy resin compositionshould have a tan δ that falls within the following defined ranges,prior to significant resin cross-linking or curing:

[0045] The resin composition used in this invention preferably has a tanδ of between about 1.2 and about 2.0, preferably between about 1.5 andabout 1.8, more preferably about 1.35, at 70° C.; or, a tan δ of betweenabout 0.7 and about 2.0, preferably between about 0.9 and about 1.8,more preferably about 1.35, at 100° C.; or, a tan δ of between about 0.5and about 1.7, preferably between about 0.7 and about 1.5, morepreferably about 1.35, at 140° C.

[0046] Preferably, the tan δ of the resin composition is from about 0.5to about 2.0, more preferably between about 1.0 and about 1.8, mostpreferably about 1.35, throughout the elevated temperature range of fromabout 70° C. to about 140° C., or if the minimum viscosity temperatureis below 140° C., the range of from about 70° C. to the minimumviscosity temperature.

[0047] More preferably, prior to significant resin cross-linking orcuring, the resin composition has a tan δ of between about 1.0 and about2.0, more preferably between about 1.2 and about 1.8 at about 70° C.;between about 0.7 and about 2.0, more preferably between about 1.0 andabout 1.7 at about 1 00° C.; and, between about 0.5 and about 2.0, morepreferably between about 0.6 and about 1.7 at about 140° C., or at theminimum viscosity temperature, if the minimum viscosity temperature isbelow 140° C.

[0048] Typically, the preferred resin composition used herein can have aminimum complex viscosity of 100 to 50,000 centipoise when heated at thetypical processing rates used in the industry (0.5-5° C./min). Further,it is preferred that the resin composition comprises an epoxy resin andhas an average epoxy functionality of greater than 2.0.

[0049] The value of tan δ is determined for purposes of the presentinvention using a Rheometrics Scientific RDA-II or comparableinstrument, operated in the oscillation mode using parallel plates witha diameter of 40 mm and a gap of between 0.5 and about 1 mm. Strain isset at 40% and torque is adjusted within the range of 5 to 1200 g·cm(0.49-117.7 mN·m). Frequency is 10 rad/s (1.59 Hz). Those skilled in theart will recognize that the critical parameters are the oscillationfrequency and strain. The other parameters can be adjusted to provideidentical measurement conditions using different geometries (and/orusing different instruments which operate on the same principle).Measurements are taken within the temperature range of 70 to 140° C. ata heating rate of 2° C./min.

[0050] It has been discovered in accordance with the present inventionthat, when the prepreg of the present invention made with an epoxy resincomposition having a tan δ value within the ranges as defined above, theporosity in the finished composite structure prepared from the prepregis reduced as compared to that in a sandwich structure manufactured froma prepreg prepared with a conventional epoxy resin. As used herein,porosity refers to the percentage of the area occupied by the pores orvoids in a cross section of a finished fiber reinforced composite panelbased on the total area of the cross section. Generally, porosityincludes all pores or voids formed in cured laminates of the finishedfiber reinforced composite structure, both in the inter-ply zones, i.e.,the zones between plies, and the intra-ply zones, i.e., the zones withinone cured prepreg ply.

[0051] As will be apparent to the skilled artisan, in the resincomposition of the present invention, the tan δ and the flow rate can becontrolled by many different methods. For example, they can becontrolled by varying the types and relative content of the solid and/orliquid elastomers, and the extent of the epoxy-elastomer reaction, i.e.,the selection of the types and extent of functional groups of the solidor liquid elastomers capable of reacting with the epoxy resins in theresin composition and/or by selection of resin and curing compositionshaving appropriate viscosities.

[0052] The hardenable resinous material can be applied to the fabricusing any conventional method known in the art. For example, the resincan be applied by a solvent method or a solventless method, both ofwhich are known in the art. In the solvent method, which is also knownas tower method or wet method, the prepreg is prepared by impregnatingthe fabric with the resin composition that is dissolved in anappropriate solvent. For example, a resin composition can be dissolvedin a solvent in a resin tank and applied to a fabric as the fabricpasses through the solution in the tank. In a solventless method (alsoknown as hot-melt method), the resin is formed into a resin film on anappropriate substrate and is subsequently transferred onto the fabric byheat and pressure. For example, a solvent free polymeric resin system isprepared and a paper substrate is coated with a resin film from thepolymeric resin. A prepreg is then prepared by transferring resin filmsfrom two paper substrates onto the top and bottom surfaces of a fabric,while two resin films sandwiching the fabric are heated andconsolidated.

[0053] The resin should be applied to the fabric such that the fabric issubstantially impregnated. The resultant prepreg should have a resincontent of from about 20 to about 60 percent by weight, preferably about30 to about 50 percent by weight, more preferably about 35 to about 45percent by weight, and most preferably about 38-42 percent by weight,based on the total weight of the prepreg. The prepreg thus prepared canthen be wound onto a roll or the like for storage or shipping.Optionally, the prepreg can be subjected to various post-impregnationtreatments. For example, as is known in the art, the prepreg can bepolished (which is also known as being “calendered” or “subjected to“compaction”) to make the surface of the prepreg more even and to reducethe openness of the prepreg. Typically, a prepreg can be polished bypressing the prepreg under a pressure of about 40 to 80 psi at atemperature of about 120-160° F.

[0054]FIG. 2 illustrates a prepreg made by impregnating a fabric of FIG.1 with a resin composition. FIG. 2A is a diagram showing the crosssectional view of the prepreg of FIG. 2 taken along a weft tow strand 24(line 2A). As indicated in FIG. 2A, the prepreg has a maximum thicknessT_(p). The warp tow strand 22 has a maximum tow width W_(warp) and amaximum tow thickness of T_(warp). Likewise, the weft tow strand 24 alsohas a maximum tow width W_(weft) and a maximum tow thickness of T_(weft)(not shown). The distance o between two adjacent warp tow strands 22indicates one dimension of the opening 26 formed by two adjacent wefttow strands 24 and two adjacent warp tow strands 22.

[0055] As used herein, “tow aspect ratio” of a prepreg is defined to bethe ratio between the maximum cross sectional width of a tow strand, W,and the maximum cross sectional thickness of the tow strand, T, asmeasured in a prepreg, that is, W/T. It should be understood that towaspect ratio is affected by the various steps in, e.g., towmanufacturing, fabric preparation from tows, prepreg manufacturing fromthe fabric, the post-impregnation treatment of the prepreg, as well asthe steps in the preparation of composite sandwich structures from theprepreg. Typically, a generally round tow (See FIG. 2B) in a prepregwill have a low tow aspect ratio. Conversely, a tow that is flattenedduring the manufacturing processes will have a large tow aspect ratio.As used herein, the term “prepreg average tow aspect ratio” is theaverage of the tow aspect ratio of the warp tow strands and the weft towstrands in the prepreg. Typically, measurements taken from at least 25to 50 different tows in each direction should be used in arriving at theaverage tow aspect ratio of a prepreg. Likewise, the term “average towaspect ratio in one direction” refers to the average tow aspect ratio ofthe tow strands in one direction of a two-dimensional woven fabric.

[0056] In accordance with this invention, different methods can be usedto manipulate or modify the cross sectional tow shape, and therefore thetow aspect ratio of a fiber tow strand of a prepreg following weaving,impregnation and optionally post-impregnation treatment. Examples ofsuch methods include, but are not limited to, sizing the tows prior toweaving, tow twisting, tow twisting and untwisting, varying the crosssectional shapes of the individual filaments in the tows and/oremploying various other modifications of the tow forming process.

[0057] It has been found that irregularities in cross sectional shapesof the individual fibers or filaments in the tow can increase theentanglement of the filaments and reduce tow aspect ratio. Inparticular, filaments having specific cross sectional shapes can beselected to vary the tow aspect ratio. For example, it has been foundthat individual filaments having a kidney or pea shape in cross sectionwill generally form a rounder fiber tow having a smaller aspect ratiofollowing weaving and impregnation as compared to an otherwise identicaltow in which each filament has a round cross section.

[0058] Another method for modifying the tow construction and tow aspectratio is by twisting and/or twisting untwisting the tow to a desiredextent. Twisting processes are well known in the art. As used herein, a“twisted tow” means a tow that is subjected to a twisting process duringthe tow forming but which is not subjected to an untwisting process asdescribed below. Typically, twisted tows have a rounder shape and thus asmaller fiber tow aspect ratio following weaving. The degree oftwisting, i.e., the number of turns per unit length, can vary with theshape, or tow aspect ratio desired. For example, twisted carbon fibertows having about 15 twists per meter are generally available and can beused in this invention. An “NT” tow, that is, a tow that is nevertwisted, can also be used in this invention. “Untwisted” tows or “UT”tows can be formed by first twisting a filament bundle to a desireddegree and then untwisting, i.e., winding the twisted fiber filamentbundle or tow in the opposite direction, to a desired degree. Typically,NT tows have a relatively flatter tow shape and thus a relatively largertow aspect ratio as compared to both twisted tows and UT tows.

[0059] Twisting or untwisting can be done either before or after thecarbonization of the precursor filaments as will be apparent to those ofskill in the art. Different procedures can be used. For example,precursor filaments can be carbonized after twisting, and then untwistedthereafter. Alternatively, a precursor filament bundle can be carbonizedin the never twisted condition. Thereafter, twisting can be optimallyperformed on the carbonized tow, as desired.

[0060] Another method of arriving at a desired cross sectional shape ofa tow strand is by sizing the tow. As is known in the art, sizing refersto a process which includes coating or impregnating a tow with a sizingagent, and drying the sizing agent, thus fixing or substantially fixingthe shape of the cross sectional shape of the tow. Any conventionalsizing agents can be used. Preferably, an epoxy-based orepoxy-compatible sizing agent is used. As is known in the art, anepoxy-based sizing agent may optionally contain, e.g., a weak epoxyresin, and additives such as polyethylene glycol, water-solublepolyurethane resin, polyvinyl formal resin, nonionic surfactant and/or acationic surfactant. Various drying methods can be used after the sizingagent is applied, including, e.g., drum drying, air drying, and airblowing. It is noted that drying methods may also affect the crosssectional tow shape and, thus the tow aspect ratio. Typically, sizing isdone after the carbonization process.

[0061] In addition, a predetermined, substantially stable crosssectional shape can be imparted to the tow by passing the tow during thetow preparation process, through a specially designed shaping die. Theshape of the die can be designed to be the shape of the desired towcross section. Prior to contacting the tow with the die, the tow can besubjected to a sizing operation so that the shaping operation willsubstantially fix the cross sectional shape of the tow.

[0062] In order to achieve a particular fiber tow construction or shapeand a particular fiber tow aspect ratio, different methods can beutilized individually or in various combinations. Some minor degree ofexperimentation may be required to determine which method or combinationof methods is most effective, this being well within the capability ofone skilled in the art once apprised of the present disclosure. Forexample, sizing operation can be performed on twisted, NT or UT tows. Inpreparing an UT tow, sizing can be done either before the twisting,after the twisting but before the untwisting, or after the twisting, orin various combinations thereof.

[0063] As used herein, “prepreg openness” refers to the percentage ofthe area in the prepreg which corresponds to the openings in theprepreg, e.g., openings 26 in FIG. 2. Such openings result from theopenings of the fabric used in the prepreg, as indicated by openings 16in FIG. 1. However, it should be understood that the prepreg opennesscan be different from the openness of the fabric used in making theprepreg. Typically, when the resin used in the prepreg is transparent,prepreg openness corresponds to the percentage of the area through whichlight can transmit. Thus, the extent of prepreg openness can readily bedetermined by optical inspection and measurements as detailedhereinafter.

[0064] Referring back to FIG. 2A, the maximum thickness Tp representsthe maximum thickness in the area where a warp tow strand 22 intersectsand floats over a weft tow strand 24. Generally, in measuring thethickness of a prepreg, a thickness gauge with a presser foot covering arelatively large area, e.g., one square inch, is used. As used herein,the term “prepreg thickness” means the average of at least 25-50thickness measurements taken in different representative areas of aprepreg.

[0065] For purposes of the present invention, the measurements todetermine prepreg thickness, openness, and prepreg tow aspect ratio aretaken after the sample prepregs have been subjected to a conditioningtreatment according to which the prepreg samples are subjected to acompacting pressure of about 45 psi at a temperature of 160° F. forabout three minutes. The compacting pressure is exerted using apneumatic-driven hot press, which contains two parallel platens withheating elements inside controlled by a temperature controller. Theup-and-down movement of these two platens is driven by air pressurecontrolled by an air regulator. This conditioning treatment is performedin order to generate uniform characteristics of the prepreg beingtested. In addition, it is intended to simulate the conditions that aprepreg is subjected to during the curing process for making a fiberreinforced composite structure.

[0066] In accordance with the present invention, it has been discoveredthat when a prepreg is used in a fiber reinforced composite structure, anumber of prepreg properties are related to the degree of core crush ofthe composite structure. In particular, it has been discovered thatthree properties of a prepreg, namely the prepreg thickness, prepregopenness and the average tow aspect ratio of a prepreg, eitherindividually or in combination, are highly determinative the degree ofcore crush. While not wishing to be bound by any theory, it is believedthat such prepreg properties contribute to the roughness of the prepregsurface and thus the friction force between prepreg plies in the lay-uplaminates during the curing process for making a composite sandwichstructure. An increase in roughness and thus friction force leads to thereduction in the degree of core crush. As used herein, the “degree ofcore crush” is defined as the percentage of the area in the sandwichstructure crushed during the curing process as discussed below in detailin connection with the method for testing a prepreg in a core crushpanel.

[0067] In particular, it has been found that for prepregs made withfabrics having the same areal weight, i.e., basis weight, when theprepreg thickness is greater, the degree of core crush is lower. It hasalso been discovered that when the prepreg openness is increased, thedegree of core crush is reduced. Further, the smaller the average fibertow aspect ratio of a prepreg, the lower the degree of core crush thatis observed.

[0068] Thus, in accordance with the first embodiment of the presentinvention, a prepreg is provided which when used in a fiber reinforcedcomposite structure, greatly reduces the degree of core crush. Inparticular, when the prepreg of this invention is used in making a fiberreinforced composite structure, the degree of core crush is preferablyno greater than about 15%, more preferably no greater than about 10%,and most preferably no greater than about 5%.

[0069] In accordance with the first embodiment, the prepreg of thisinvention has a prepreg openness of, at least about 1.0%, preferably atleast about 2.0%, more preferably at least about 2.5%, even morepreferably at least about 3.0%, and most preferably at least about 3.8%as determined by the optical inspection method described hereinafter.

[0070] However, typically the prepreg openness will be less than about10.0%, more preferably, less than about 6.0%.

[0071] The prepreg has a prepreg thickness of at least about 0.220 mm,preferably at least about 0.245 mm, more preferably at least about 0.250mm, more preferably at least about 0.260, even more preferably at leastabout 0.265 mm, and most preferably at least about 0.270 mm.

[0072] Alternatively, the prepreg average tow aspect ratio is less thanabout 15.5, preferably less than about 14.0, more preferably less thanabout, 13.0, even more preferably less than about 12.5, and mostpreferably less than about 11.5.

[0073] It is noted that in this embodiment of the prepreg of thisinvention, the prepreg will have at least one of the above-describedthree properties. That is, at least one of the three properties of theprepreg, namely prepreg openness, prepreg thickness, and prepreg towaspect ratio falls with the corresponding range as described above.Preferably at least two of the above three requirements are met in thisembodiment of the invention. Most preferably, all three requirements aremet by the prepreg.

[0074] The prepreg in accordance with this first embodiment of thepresent invention can be prepared by the methods as described in detailabove. Typically, different fabrics may be impregnated with a resincomposition as described above to prepare a prepreg. The properties ofthe prepreg, namely, prepreg openness, prepreg thickness, and averagetow aspect ratio are measured and compared with the above-definedranges.

[0075] In accordance with the second embodiment of the prepreg of thisinvention, a fabric having a complex weave is used in the prepreg toreduce both core crush and porosity. The fabric is of the same type asthe fabric 10 generally described above, i.e., a two-dimensional wovenfabric, preferably a biaxially woven fabric with the warp tow strandscrossing the weft tow strands at a substantially right angle. However,for purpose of this second embodiment of the present invention, the towstrands in one direction, e.g., warp or weft, have a different towconstruction and a different tow aspect ratio, from the tow strands inthe other direction, e.g., weft or warp. By “different tow construction”it is intended to mean that the tow strands in the two directions aredifferent in nature, i.e., they are formed either from different fiberfilaments or by different tow manufacturing processes, or throughmanipulation of the weaving processes, or a combination of theseprocesses. For example, the tow strands in one direction can be twistedtows while the tow strands in the other direction being never twisted(NT) tows. In another example, the tow strands in one direction can havea total filament count of about 3,000 while those in the other directionhaving a total filament count of, e.g., from about 4,000 to about12,000. Since the average tow aspect ratios of the tow strands in thetwo different directions are different in this embodiment of theinvention, the tow strands in the two directions typically havedifferent cross sectional shapes or dimensions. For example, tows in onedirection can be rounder and the tows in the other direction can beflatter, as is apparent from the above detailed description related totow aspect ratio.

[0076] While not wishing to be bound by any theory, it is believed thatlarge openness may cause a large amount of voids to form in the curedprepreg plies of a composite sandwich structure due to air bubblestrapped within the plies during the curing process. In addition, it isbelieved that when prepreg thickness is increased, porosity may beincreased due to the weave peak/valley mismatches between adjacentprepreg plies. It is also believed that when prepreg average aspectratio is too small, prepreg openness can be great enough so as toexacerbate the porosity problem. It has been found that prepreg opennesscorrelates, to some extent, to porosity, and prepreg openness can beindicative as to the degree of porosity. Again while not wishing to bebound by any theory, it is believed that, because hybrid tows areemployed in the second embodiment of the present invention, the frictionforce between the prepreg plies is increased as compared to theconventional prepregs known heretofore in the art, while the prepregopenness can be kept low. Thus, in a fiber reinforced compositestructure prepared with a prepreg of this embodiment, both core crushdegree and porosity are low.

[0077] FIGS. 3 illustrates the fabric design of a woven fabric 30, whichis an example of the suitable fabrics for use in the prepreg of thesecond embodiment of this invention. FIG. 4 shows a prepreg 40 made byimpregnating fabric 30 with a hardenable resin composition. Fabric 30 isof the same type as fabric 10 shown in FIG. 1. As illustrated in FIGS.3, 4, and FIGS. 4A and 4B, which are cross sectional views along 4A-4Aand 4B-4B in FIG. 4 respectively, the weft tow strands 34 and 44 have adifferent cross sectional tow shape from the warp tow strands 32 and 42.

[0078] Referring now to FIG. 4A, in the prepreg 40, the weft tow strands44 have a maximum tow width W_(weft) and a maximum tow thicknessT_(weft). Therefore, weft tow strands 44 have a fiber tow aspect ratio W

[0079] weft/T_(weft). Likewise, as shown in FIG. 4B, warp tow strands 42have a maximum tow width WWarp and a maximum tow thickness T_(warp), andthus a fiber tow aspect ratio W_(warp)/T_(warp).

[0080] In accordance with this aspect of the present invention, theaverage tow aspect ratio of the tow strands in one direction is nogreater than about 13.0, preferably no greater than about 12.5, morepreferably no greater than about 12.0, most preferably no greater thanabout 11.0, while the average tow aspect ratio of the tow strands in theother direction being at least about 13.5, preferably at least about14.0, more preferably at least 14.5, and most preferably at least about15.5.

[0081] In addition, the prepreg according to the second embodimentpreferably has a prepreg openness of no greater than about 5.0%, morepreferably no greater than about 4.0%, even more preferably no greaterthan about 3.5%, and most preferably no greater than about 3.0%.

[0082] It is also preferred that the prepreg of this second embodimenthas a prepreg thickness of from about 0.230 mm to about 0.300 mm,preferably from about 0.240 mm to about 0.290 mm, more preferably fromabout 0.250 mm to about 0.280 mm, most preferably from about 0.260 mm toabout 0.270 mm.

[0083] In this embodiment of this invention, the fiber tow strands inboth warp and weft directions can have the same filament count in eachfiber tow strand. Alternatively, the filament counts for the tow strandsin two directions can be different, e.g., about 3,000 in one directionwhich has the lower average tow aspect ratio, and above 3,000 but nogreater than about 18,000, preferably no greater than about 12,000 inthe tow strands having the higher average tow aspect ratio. When thefiber filament counts are different in the two directions, preferablythe tow strands having a greater tow aspect ratio have the greater fiberfilament count.

[0084] As discussed above, there are different methods to manipulatefiber tow strands to arrive at the desired tow constructions and towaspect ratios, all of the methods being applicable in this aspect of thepresent invention.

[0085] Unexpectedly, when this second embodiment of the prepreg of thisinvention is used in making a fiber reinforced composite structure, boththe degree of core crush and the porosity can be low. Typically, thedegree of core crush is less than about 15%, preferably less than 10%,and more preferably less than 5%. In the meantime, the porosity in thecomposite structure is low and is substantially within the satisfactoryrange for aviation use.

[0086] In accordance with yet another embodiment of the presentinvention, there is provided a prepreg which comprises in theresin-impregnated fabric, tow strands having a predeterminedsubstantially stable, non-round cross sectional shape. Typically suchtow strands have an average fiber tow aspect ratio of from about 8.0 toabout 18.0, preferably from about 10.0 to about 16.0, even preferablyfrom about 12.0 to about 14.5, more preferably from about 12.5 to about14.0, even more preferably from about 13.0 to about 14.0, and mostpreferably from about 13.0 to about 13.5. In addition, the prepregopenness is preferably no greater than about 5.0%, more preferably nogreater than about 4.0%, even more preferably no greater than about3.0%, and most preferably no great than about 2.0%. Typically, theprepreg has a prepreg thickness of from about 0.240 mm to about 0.300mm, preferaby from about 0.250 mm to about 0.275 mm, more preferablyfrom about 0.255 mm to about 0.270 mm, and most preferably from about0.260 mm to about 0.265 mm. It is noted that the prepreg properties canvary within the above ranges with tow strands having different filamentcounts. For example, if tows having a greater filament count, e.g.,12,000 are utilized, it is expected that the preferred prepreg thicknesswould be greater, e.g., at least 0.280 mm, and that the preferredopenness would be, e.g., about 4.0%. It has been discovered that theprepreg in accordance with this third embodiment of the presentinvention can substantially reduce both core crush and porosity in fiberreinforced composite structures.

[0087] By “non-round cross sectional shape” it is intended to mean thatthe cross sectional shape of the tow strand is not round in that thecross-section includes one or more tip ends that taper to a generallypointed end, as opposed to the rounded ends and curved tapering of anoval or similar continuous round shape. Such non-round cross-sectionallows the tow to be sufficiently wide in shape and yet sufficientlythick near the tow centerline to achieve a high surface friction whilereducing openness. Thus, two requirements should be met: first, in agiven tow strand having a fixed total cross sectional area, the towstrand should be made such that a desired average tow aspect ratiowithin the above-described ranges is achieved. Second, the crosssectional tow shape should be arranged such that the greatest crosssectional width of the tow strand is substantially achieved while thedesired average tow aspect ratio is complied with. Generally, so long asthese two requirements are met, the cross section of the tow can be inany non-round shape. For example, as illustrated in FIG. 2C, the crosssectional shape of the tow strand 22′ can be in an eye shape or spearshape, i.e., the two lateral tow tips along the width of the tow aresubstantially extended in the lateral direction, and are preferablysharply pointed ends, while the tow thickness at the center isnevertheless maintained at a certain desired level so as to meet the towaspect ratio requirement as described above. To give another example,the cross sectional tow shape can be in a diamond shape. It is notedthat the perimeter or circumference of the cross section of the towstrand need not be substantially smooth.

[0088] The cross sectional tow shape should be substantially stable. Inother words, once the tow strands are formed, the cross sectional shapeof the tow strands are substantially set and are maintained in thesubstantially same shape during the subsequent processes of, e.g.,fabric preparation from tows, prepreg manufacturing from the fabric, thepost-impregnation treatment of the prepreg, as well as the steps in thepreparation of composite structures from the prepreg.

[0089] As is apparent from the discussion above in relation to themethods for modifying tow constructions and tow aspect ratio, manydifferent methods, either individually or in various combinations, canbe used to prepare a tow strand having a predetermined substantiallystable non-round cross sectional shape. Without repeating the details asdescribed above, such methods include, but are not limited to, extrusiontow strands through a specially designed die having the same shape asthe desired cross sectional shape of the tow, sizing the tows prior toweaving, tow twisting, tow twisting and untwisting, varying the crosssectional shapes of the individual filaments in the tows, and variousother modifications of the tow forming process.

[0090] Except for the special tow constructions and the prepreg opennessrequirement as specified above, the prepreg according to this embodimentis of substantially the same type as the prepreg illustrated in FIG. 2.FIG. 2C illustrates a cross sectional view of a prepreg in accordancewith this third embodiment of the invention. It is noted that, ascompared to the tow strands of the embodiment shown in FIG. 2B, whilethe tow thickness in FIG. 2C is comparable to that shown in FIG. 2B, thetow width of the tows in this third embodiment is substantially greaterthan that shown in FIG. 2B. As a result, the prepreg openness is reducedwhile the prepreg thickness is not substantially decreased.

[0091] In accordance with yet another embodiment of this invention,there is provided a method for evaluating the core crush resistanceproperties of a prepreg for use in a fiber reinforced compositestructure. As described above, it has been discovered in accordance withthis invention that the prepreg thickness, average tow aspect ratio, andprepreg openness of a prepreg used in the curing process for making afiber reinforced composite structure all correlate with the degree ofcore crush during the curing process. Accordingly, the method forevaluating the core crush resistance property of a prepreg includesdetermining the prepreg thickness, the average tow aspect ratio and/orprepreg openness, and comparing the obtained results to a set ofpredetermined values. Typically, values for prepreg thickness, averagetow aspect ratio, and/or prepreg openness corresponding to differentcore crush degrees are obtained with different types of fabrics and/orresins. Such values can then be used as “predetermined values.” Bycomparing the values for prepreg thickness, average tow aspect ratio,and/or prepreg openness measured from a particular prepreg beingevaluated to the predetermined values, the range within which the degreeof core crush in a curing process using the prepreg will likely fall canbe predicted. It is noted that the predetermined values can vary whensubstantially different fabrics and/or resins are used in preparing theprepreg. By “substantially different fabrics” it is intended to mean,e.g., substantially different areal weights are used, or made fromdifferent types of fibers having substantially different mechanicalproperties, etc. However, an ordinarily skilled artisan apprised of thepresent invention should be able to determine the “predetermined values”for prepregs made from any types of fabrics and/or resins.

[0092] By way of example, for prepregs made from woven fabrics having anareal weight of from about from about 150 to about 400 grams per squaremeter, preferably from about 150 to about 250 grams per square meter,more preferably from about 180 to about 205, most preferably from about185 to 201 grams per square meter, and consisting essentially of carbonfiber tow strands impregnated with a hardenable resin, a set ofpredetermined values have been determined. The values for prepregthickness and average fiber tow aspect ratio and the correspondingdegree of core crush are shown in FIGS. 5 and 6.

[0093] In FIGS. 5 and 6, plain weave fabrics having an areal weight inthe range from about 185 to about 201 grams per square meter wereprepared from different types of carbon fiber tows. The fabrics wereimpregnated with either of the two resin compositions described inExample 1 below. Partly because different fiber tows and differentresins were used, the prepregs thus prepared had different prepregthickness and average tow different aspect ratios. As illustrated inFIGS. 5 and 6, both prepreg thickness and average tow aspect ratiocorrelate with the degree of core crush. In FIGS. 5 and 6, ST stands fora prepreg made from a fabric prepared from carbon fiber tows which havea filament count of 3,000 and are twisted for 15 turns per meter; NTstands for a prepreg made from a fabric prepared from carbon fiber towswhich have a filament count of 3,000 and are never twisted; UT standsfor a prepreg made from a fabric prepared from carbon fiber tows whichhave a filament count of 3,000 and are twisted for 15 turns per meterbefore sizing and untwisted for 15 turns per meter after sizing; HybridST/NT stands for a hybrid prepreg as described above prepared from afabric having ST fiber tows in one direction and NT fiber tows in theother direction.

[0094] As an example, in a prepreg being evaluated, if the average fiberaspect ratio is no greater than about 13.5 and the prepreg thickness ismore than about 0.260 nun, the core crush degree in a curing processusing the prepreg can be predicted to be below about 10%. In addition,it has also been determined that when the prepreg openness is greaterthan about 3%, the core crush degree is normally below about 10%.

[0095] In the method of the present invention, although a value of onlyone of the three properties, namely prepreg average tow aspect ratio,prepreg thickness, prepreg openness, may be sufficient for predictingthe core crush degree, it is preferred that the values for at least twoof the three properties are determined and compared to the predeterminedvalues, preferably one of the two being the prepreg average tow aspectratio. More preferably, the values for all three properties aredetermined and compared to the predetermined values respectively.Although certain discrepancies may result from this method, generallythe accuracy of the prediction based on this method can be above about80%, especially when all three properties are examined. Thus, the methodaccording to the present invention can be very useful in selecting corecrush resistant prepregs for preparing fiber reinforced compositestructures, especially those for aviation use.

[0096] In accordance with yet another embodiment of the presentinvention, a fiber reinforced composite structure is provided which isprepared using a prepreg of this invention as disclosed above. Fiberreinforced composite structures are well known in the art. Differentmethods for reducing core crush, e.g., various tiedown methods are knownin the art in making the present reinforced composite structure of thisinvention using the prepreg provided in the present invention,advantageously the prior art methods and devices for reducing core crushcan be omitted, and yet a core crush degree of less than about 15%, morepreferably less than about 10%, and most preferably less than about 5%can be achieved. Of course, if it is desirable, those prior art devices,such as tiedown devices, for reducing core crush can also be used inmaking the fiber reinforced composite structure of this invention.

[0097] The fiber reinforced composite structure can be prepared by anysuitable methods known in the art. Typically, plies of the prepreg ofthis invention are laid up in laminates on one or both sides of alightweight core or honeycomb core formed of e.g., aluminum, Nomex®,fiberglass, etc. The lay-up is then autoclaved in a vacuum bag placed inan autoclave under conditions such that the prepregs are cured andadhered to the honeycomb core. For example, U.S. Pat. No. 5,685,940,which is incorporated herein by reference, discloses an improved methodfor making a fiber reinforced composite structure, which can be used inthe present invention.

[0098] The invention is further demonstrated in the following examples,which are used only for purpose of illustration but not to limit thescope of the present invention.

[0099] In accordance with the invention, prepreg and compositeproperties are evaluated using the testing methods described below. Itshould be noted that in these methods, the measurements indicated hereinfor prepreg thickness, openness, and prepreg tow aspect ratio are takenafter the sample prepregs have been subjected to a conditioningtreatment according to which the prepreg sample is subjected to acompacting pressure of 45 psi using a pneumatic-driven hot press at 160°F. for three minutes. This conditioning treatment is performed in orderto generate uniform characteristics of the prepreg being tested. Inaddition, it is intended to simulate the conditions that a prepreg issubjected to during the curing process for making a fiber reinforcedcomposite sandwich structure.

Method of Measuring Prepreg Openness

[0100] A piece of prepreg is laid flat under a microscope withtransmitted light passing through the prepreg from under. No force orpressure is applied on this prepreg. An image, showing the prepreg andits openness as black and very light gray respectively, is viewed by avideo camera (attached to the microscope) which transmits the image indetail form to the image grabber of a PC computer. The image is thenconverted into a rectangular array of integers, corresponding to thedigitized gray level of each picture element (pixel). An image analysisprogram such as Optimas 6.2, in the PC, or the like is used to processthis digital image information and represents it in the form of a graylevel histograms. This histograms summarizes the gray level content ofthe image. In this case two distinct groups, corresponding to theprepreg and openness, can be found in the histograms. These two groupscan be easily separated by a simple thresholding process. The prepregopenness is thus obtained as the percentage ratio between the number ofpixels corresponding to the group associated with openness and the totalnumber of pixels in the image.

[0101] In order to obtain more accurate and representative results,openness is measured at a very low magnification (5× or under). Eachimage contains at least ten fiber tow strands in each direction. Severalpieces of prepreg randomly chosen from different locations of a prepregroll are measured, and an average is taken as the openness of theprepreg.

Method of Measuring Prepreg Thickness

[0102] Typically, prepreg thickness is measured by a thickness gaugewith presser foot covering a relatively large area on the prepregsurface. For example, a test setup similar to ASTM D1777-96 (StandardTest Method for Thickness of Textile Materials) is used. The apparatuscontains a thickness gauge with a one inch square presser foot and a 5pounds dead weight on it. This is equivalent to approximately 5 psipressure applied to the specimen when a measurement is taken. Severalpieces of prepreg are randomly chosen from different locations of aprepreg roll and several measurements are taken for each piece. Theaverage of the all the measurements from one prepreg roll can be takenas the prepreg thickness.

Method of Measuring Fiber Tow Width and Thickness in Prepreg

[0103] The width of a fiber tow strand can be determined by thelight-transmission method as described above for determining prepregopenness, except that high magnification is used in this case toincrease the measurement resolution and fiber tow is magnified in such away that its width covers most part of the image. The same image program(Optimas 6.2) is calibrated before measurement through an acquired imageof a very fine ruler positioned at the same height as the measuredobject. By using the Optimas software to draw a line on a knowndimension of the ruler, the programs can “memorize” the length of thisline and use this information as a base for any other lengthmeasurements under the same condition. Fiber tow width is then measuredby simply drawing a line to cover the entire fiber tow width and theprograms can automatically calculate the length of this line based onthe saved calibration data.

[0104] Again, several pieces of prepreg are randomly chosen fromdifferent locations of the prepreg roll and each piece has severalmeasurements. Final number is based on the average of all measurements.

[0105] Similarly, to measure the fiber tow thickness, a piece of prepregis carefully cut with surgical scissors along the centerline of fibertows. The thickness is then determined in a method similar to that formeasuring the width of a fiber tow strand. Again, no force or pressureis applied. Very high magnification is necessary in this case toincrease the measurement resolution. Light sources can be positioned insuch a way that the 90-degree layer becomes white in contrast to dark0-degree layer for each measurement. Measurements are made again in bothwarp and fill directions.

Testing Prepregs in Core Crush Panels

[0106] A standard core crush panel 70 as shown in FIG. 7 is usedconsisting of 28″×24″ composite skins and a 24″×20″ Nomex core (⅛″ cellsize, 0.5″ thick, 3.0 pcf, e.g., Hexcel Corporation HRH-10, orequivalent) with a 20° chamfer angle. FIG. 7A is a cross section view ofthe core crush panel 70, illustrating the lay up of the core crush panelprior to being cured. The types, directions, and dimensions of prepregplies, as well as those of the honeycomb core are specified in Table I.Additionally, layers of adhesive are typically between the core andprepreg plies 75 and 77, and below ply 72 (not shown). TABLE I PLYNUMBER TYPE DIRECTION DIMENSION 72 FULL 0°-90° 28″ × 24″ 73 DOUBLER ±45°28″ × 24″ Picture-frame opening to allow 1.25″ back from core mold point74 DOUBLER 0°-90° 28″ × 24″ Picture-frame opening to allow 0.75″ backfrom core mold point 75 FULL ±45° 28″ × 24″ CORE TRANSVERSE 20″ × 24″2.0″ upper edge radius angle radius fair from 2.0″ to 0.15″ 76 FILLER0°-90° 2 Pcs-2″ × 26″ 2 Pcs-2″ × 24″ 77 FULL ±45° 28 25″ × 24.25″ 78DOUBLER 0°-90° 28.25″ × 24.25″ Picture-frame opening to allow 0.75″ backfrom core mold point 79 DOUBLER ±45° 28.25″ × 24 25″ Picture-frameopening to allow 1.25″ back from core mold point 80 FULL 0°-90° 28.25″ ×24.5″

[0107] To cure the panel 70, the panel is placed in a vacuum bag. Thevacuum bag and the panel therein are then placed in an autoclave. Thebag is evacuated and cured under pressure at an elevated temperature.The curing cycle includes the following steps: (1) applying vacuum of3.9 psia (27 kPa) minimum to the vacuum bag; (2) pressurize theautoclave to 413 kPa (45 psia) (including venting the vacuum bag toatmosphere when the autoclave pressure reaches 20 psia); (3) raising thetemperature inside the autoclave at a rate of 1-5° F./min; (4) curingthe panel at 355° F. for 2 hours (under the pressure established in step2); (5) cooling down at a rate of 5° F./min, and (6) following curing,when part temperature has fallen to 140° F., relieving the pressure,removing the vacuum bag and debagging.

[0108] The dimensions of the cured core crush panel are measured asshown in FIG. 8. X is the displacement of the center of the core sidefrom its original position. L represents the original length of coreside. The crushed area A is calculated according to the formula:$A = {\sum\limits_{n = 1}^{4}{{2/3} \cdot X_{n} \cdot L_{n}}}$

[0109] The degree of core crush in percentage is determined by thefollowing formula:

Percent core crush=100×A/480.

[0110] To examine interlaminar porosity, a standard core crush panel iscut along the line marked 7A-7A in FIG. 7. The cross section is examinedvisually. One linear inch of the exposed edge in a ply area whichappears to have the highest porosity is polished with a diamond polish(e.g., 0.3-micron diamond polish) and examined under 50× magnificationfor internal porosity. Five specimens are measured and an average istaken as the porosity.

[0111] In the following example, prepregs were made in accordance withthe present invention. The sample prepregs were subjected to aconditional treatment: a compaction pressure of 45 psi was applied onthe sample prepregs at 160° F. for three minutes to partially simulatethe compaction imported to the panel in the autoclave during the curingprocess. The prepreg openness, thickness, and fiber tow aspect ratiowere then measured from the sample prepregs. The prepregs were tested ina core crush panel for their core crush resistance and porosity.

EXAMPLE

[0112] Fabrics were prepared from ST (standard twisted), UT (untwisted),NT (never twisted) fiber tows, or both ST and NT fiber tows (HybridST/NT, i.e., ST in warp direction and NT in weft direction)respectively. As used herein, ST tows stand for carbon fiber tows whichwere twisted for 15 turns per meter during the manufacturing process; NTtows are carbon fiber tows which were never twisted during the towmanufacturing process; UT tows are carbon fiber tows which were twistedfor 15 turns per meter before sizing and untwisted for 15 turns permeter after sizing. Each tow had a total carbon fiber filament count ofabout 3,000. All fabrics used were plain weave with a weaving pitch of12-13 tows/inch in both warp and weft directions, and had a fabric arealweight of about 193 grams per square meter.

[0113] The fabrics were impregnated by a solution method with either ofthe following two epoxy resin compositions: Resin No. 1 comprises ofabout 67% multifunctional epoxy resins, about 8.3% solid and liquidreactive elastomers, about 20.7% of a multifunctional amine curingagent, about 1.8% of a co-curing agent, 0.1% of a catalyst, and 2.1% ofa flow control agent, i.e., fumed silica. The resin composition No. 1has a tan δ of 0.78 at 70° C. and 0.27 at 140° C.

[0114] Resin No. 2 comprises of about 67.6% multifunctional epoxyresins, about 7.4% solid and liquid reactive elastomers; 20.7% of amultifunctional amine curing agents; about 1.8% of a co-curing agent;0.1% of a catalyst, and 2.1% of a flow control agent, i.e., fumedsilica. The resin composition No. 2 has a tan δ of 1.37 at 70° C. and1.35 at 140° C.

[0115] The prepregs thus prepared were measured for prepreg thickness,openness, and fiber tow aspect ratio. Core crush panels were preparedfrom the prepregs and core crush degree and porosity were determined.The results are shown in Table II. Porosity is indicated qualitativelyin a scale of 1 to 5, where 1 represents a low level while 5corresponding to a high level. TABLE II Prepreg Prepreg Core ResinThickness Openness Fiber Tow Aspect Ratio Crush Fabric composition (mm)(%) Warp Weft Average (%) Porosity ST (comparative) No. 1 0.289 4.9 8.811.9 10.4 0.73 5 ST No. 2 0.260 3.1 10.7 15.4 13.1 5.0 1-2 UT(comparative) No. 1 0.277 1.9 11.4 12.5 12 0.59 3 NT (comparative) No. 10.244 1.7 13.5 16.2 14.9 16 1 NT (comparative) No. 2 0.224 1.7 15.5 18.517 22.3 1 Hybrid ST/NT No. 1 0.272 3.8 9.7 15.5 12.6 3.4 3

[0116] As shown in Table II, when the prepreg thickness, average towaspect ratio, and prepreg openness meet the requirements of thisinvention, the degree of core crush generally is low, i.e., lower thanabout 15%. For example, prepregs prepared from fabrics made from ST orUT fiber tows meet the prepreg thickness, average tow aspect ratio, andprepreg openness requirements, while those prepared from fabrics madefrom NT fiber tows do not meet the requirements. Consequently, as shownin Table II, the core crush degree in a fiber reinforced compositesandwich structure prepared using the former prepregs is substantiallylower than that in a fiber reinforced composite sandwich structureprepared using the latter prepregs. Although prepregs made from plainweave fabrics prepared from ST, UT, and NT fiber tows having a filamentcount of 3,000 and impregnated with a conventional epoxy resin such asResin No. 1 were known, the fact that prepreg openness, prepregthickness and average tow aspect ratio are highly determinative of thecore crush degree has never been appreciated. Accordingly, those skilledin the art prior to this invention generally expected the different towsto perform comparably.

[0117] In addition, while certain prepregs, e.g., prepregs prepared fromfabrics made of ST tows, can reduce the degree of core crush, they areusually associated with unacceptable porosity. (See ST with Resin No. 1in Table II.) Unexpectedly, when an epoxy resin such as the Resin No. 2is used which has a preferred tan δ value and an average functionalityof greater than 2, the porosity in the fiber reinforced compositesandwich structures prepared from the prepregs made from ST fabrics canbe significantly reduced to an acceptable level, as shown in Table II.It is also believed that when the same resin is applied to a fabric madefrom UT tows, the porosity will be reduced as well. However, the use ofa resin such as Resin No. 2 does not reduce core crush degree. Forexample, as shown in Table II, while the prepreg prepared from a fabricmade from NT tows impregnated with Resin No. 1 causes unacceptable corecrush, the same fabric impregnated with Resin No. 2 is associated withno less core crush.

[0118] Further, when a prepreg prepared from a hybrid fabric having STtows in one direction and NT tows in the other direction is used, bothcore crush degree and porosity are satisfactory even when a conventionalresin such as Resin No. 1 is used.

[0119] Thus, as demonstrated by the Example, the present inventionprovides prepregs which when used in making fiber reinforced compositestructures, can effectively reduce both core crush and porosity. Inaddition, because various fiber tows can be manipulated to impartdesirable prepreg properties so as to reduce core crush, this inventionallows prepreg manufacturers greater freedom in selecting fiber tows orfabrics and thus less dependence on specific specialty fiber towmanufacturers.

[0120] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A core crush resistant prepreg for use inmaking a fiber reinforced composite structure, comprising: a wovenfabric consisting essentially of carbon fiber tow strands impregnatedwith a hardenable polymeric resin composition; said fabric having anareal weight of from about 150 to about 400 grams per square meter; saidresin composition having an average epoxy functionality of greater than2.0, and a tan δ of between about 1.0 and about 2.0 at 70° C., betweenabout 0.7 and about 2.0 at 100° C., and between about 0.5 and about 2.0at 140° C. or the highest temperature of minimum resin viscosity; andsaid prepreg having an average fiber tow aspect ratio of less than about15.5.
 2. The prepreg of claim 1 , wherein said prepreg has a prepregopenness of at least about 1.0%
 3. The prepreg of claim 1 , wherein saidfabric has an areal weight of from about 150 to about 250 grams persquare meter.
 4. The prepreg of claim 1 , wherein said prepreg has anaverage fiber tow aspect ratio of less than about 14.0.
 5. The prepregof claim 1 , wherein said prepreg has an average fiber tow aspect ratioof less than about 12.5.
 6. The prepreg of claim 1 , wherein saidprepreg has a prepreg thickness of at least about 0.250 mm.
 7. Theprepreg of claim 1 , wherein said prepreg has a prepreg thickness of atleast about 0.270 mm.
 8. The prepreg of claim 1 , wherein said fabrichas a plain weave pattern.
 9. The prepreg of claim 1 , wherein saidfabric has a satin weave pattern.
 10. The prepreg of claim 1 , whereineach of said fiber tow strands has a total filament count of from about1,000 to about 12,000.
 11. The prepreg of claim 1 , wherein each of saidfiber tow strands has a total filament count of about 3,000.
 12. Theprepreg of claim 1 , wherein said fiber tow strands are untwisted towstrands.
 13. A core crush resistant prepreg for use in making a fiberreinforced composite structure, comprising: a woven fabric consistingessentially of carbon fiber tow strands impregnated with a hardenablepolymeric resin composition; said fabric having an areal weight of fromabout 150 to about 400 grams per square meter; said resin compositionhaving an average epoxy functionality of greater than 2.0, and a tan δof between about 1.0 and about 2.0 at 70° C., between about 0.7 andabout 2.0 at 100° C., and between about
 0. 5 and about 2.0 at 140° C. orthe highest temperature of minimum resin viscosity; and said prepreghaving a prepreg openness of at least about 1.0% and a prepreg thicknessof at least about 0.245 mm.
 14. A core crush resistant prepreg for usein making a fiber reinforced composite structure, comprising: a wovenfabric having a plain weave pattern consisting essentially of carbonfiber tow strands impregnated with a hardenable polymeric resincomposition comprising an epoxy resin; said fabric having an arealweight of from about 180 to about 205 grams per square meter; each ofsaid tow strands having a total filament count of about 3,000; saidresin composition having an average epoxy functionality of greater than2.0, and a tan δ of from about 0.5 to about 2.0 throughout the elevatedtemperature range of from about 70 to about 140° C.; and said prepreghaving an average fiber tow aspect ratio of less than about 13.0, and aprepreg openness of from about 1.5% to about 10.0%.
 15. A core crushresistant prepreg for use in making a fiber reinforced compositestructure, comprising: a woven fabric having a plain weave patternconsisting essentially of carbon fiber tow strands impregnated with ahardenable polymeric resin composition comprising an epoxy resin; saidfabric having an areal weight of from about 180 to about 205 grams persquare meter; each of said tow strands having a total filament count ofabout 3,000; said resin composition having an average epoxyfunctionality of greater than 2.0, and a tan δ of from about 0.5 toabout 2.0 throughout the elevated temperature range of from about 70 toabout 140° C.; and said prepreg having an average fiber tow aspect ratioof less than about 13.0, a prepreg thickness of at least about 0.260 mm,and a prepreg openness of from about 1.0% to about 10%.
 16. A core crushresistant prepreg for use in making a fiber reinforced compositestructure, comprising: a woven fabric consisting essentially of carbonfiber tow strands impregnated with a hardenable polymeric resincomposition; said fabric having an areal weight of from about 150 toabout 250 grams per square meter; said tow strands having a totalfilament count of from about 1,000 to less than about 3,000; and saidprepreg having an average fiber tow aspect ratio of less than about15.5, an average thickness of at least about 0.220 mm, and an opennessof from about 1.0 percent to about 6.0 percent.
 17. The prepreg of claim16 , wherein said resin has an average epoxy functionality of greaterthan 2.0, and a tan δ of from about 0.5 to about 2.0 throughout theelevated temperature range of from about 70 to about 140° C.
 18. Theprepreg of claim 16 , wherein said prepreg has an average fiber towaspect ratio of less than about 12.5.
 19. The prepreg of claim 16 ,wherein said prepreg has a prepreg thickness of at least about 0.240 mm.20. A core crush resistant prepreg for use in making a fiber reinforcedcomposite panel structure, comprising: a woven fabric consistingessentially of carbon fiber tow strands impregnated with a hardenablepolymeric resin composition; said fabric having an areal weight of fromabout 150 to about 400 grams per square meter; said tow strands having atotal filament count of greater than about 3,000 but no greater thanabout 18,000; and said prepreg having an average thickness of at leastabout 0.260 mm and an openness of at least about 1.5 percent.
 21. Theprepreg of claim 20 , wherein said resin composition comprises an epoxyresin having an average epoxy functionality of greater than 2.0 and atan δ of from about 0.5 to about 2.0 throughout the elevated temperaturerange of from about 70 to about 140° C.
 22. The prepreg of claim 20 ,wherein said fabric has an areal weight of from about 150 to about 250grams per square meter.
 23. The prepreg of claim 20 , wherein saidprepreg has a prepreg thickness of at least about 0.280 mm.
 24. Theprepreg of claim 20 , wherein said prepreg has a prepreg openness offrom about 3.0 percent to about 10.0 percent.
 25. A core crush resistantprepreg for use in making a fiber reinforced composite panel structure,comprising: a woven fabric impregnated with a hardenable polymeric resincomposition; said woven fabric having an areal weight of from about 150to about 400 grams per square meter and consisting essentially of aplurality of carbon fiber tow strands in a first direction interweavingwith a plurality of carbon fiber tow strands in a second direction, saidtow strands in said first direction having a different tow constructionfrom said tow strands in said second direction; wherein said prepreg hasa tow aspect ratio of no greater than about 13.0 in said first directionand a tow aspect ratio of at least 13.5 in said second direction, and aprepreg openness of no greater than 5.0%.
 26. The prepreg of claim 25 ,wherein said prepreg has a prepreg thickness of from about 0.240 mm toabout 0.280 mm.
 27. The core crush resistant prepreg of claim 25 ,wherein said tow aspect ratio in said first direction is no greater thanabout 11.0.
 28. The core crush resistant prepreg of claim 25 , whereinsaid tow aspect ratio in said second direction is at least about 15.5.29. The core crush resistant prepreg of claim 25 , wherein said prepreghas an openness of no greater than about 3.0%.
 30. The core crushresistant prepreg of claim 25 , wherein said tow strands in said firstdirection and said tow strands in said second direction have a fiberfilament count of about 3,000.
 31. The core crush resistant prepreg ofclaim 25 , wherein said tow strands in said first direction have a fibercounts of about 3,000 and said tow strands in said second direction havea fiber filament count of about 6,000.
 32. The core crush resistantprepreg of claim 25 , wherein said tow strands in said first directionare twisted tows and said tow strands in said second direction are nevertwisted.
 33. The core crush resistant prepreg of claim 25 , wherein saidtow strands in said first direction are untwisted tows and said towstrands in said second direction are never twisted.
 34. The core crushresistant prepreg of claim 25 , wherein said woven fabric has an arealweight of from about 150 to about 250 grams per square meter.
 35. Thecore crush resistant prepreg of claim 25 , wherein said resincomposition comprises an epoxy resin.
 36. The core crush resistantprepreg of claim 25 , said resin composition has an average epoxyfunctionality of greater than 2.0, and a tan δ of from about 0.5 toabout 2.0 throughout the elevated temperature range of from about 70 toabout 140° C.
 37. A core crush resistant prepreg for use in making afiber reinforced composite structure, comprising: a woven fabricimpregnated with a hardenable liquid epoxy resin composition, said wovenfabric having an areal weight of from about 185 to about 205 grams persquare meter and consisting essentially of a plurality of carbon fibertow strands in a first direction interweaving with a plurality of carbonfiber tow strands in a second direction, said tow strands in said firstdirection having a different tow construction from said tow strands insaid second direction, wherein said prepreg has a tow aspect ratio of nogreater than about 11.0 in said first direction and a tow aspect ratioof at least 15.5 in said second direction, a prepreg openness of nogreater than 3.0%, and a prepreg thickness of from about 0.250 mm toabout 0.280 mm.
 38. A core crush resistant prepreg for use in making afiber reinforced composite structure, comprising: a woven fabricconsisting essentially of carbon fiber tow strands impregnated with ahardenable polymeric resin composition, said tow strands having apredetermined substantially stable non-round cross sectional shape; saidfabric having an areal weight of from about 150 to about 400 grams persquare meter; said prepreg having an average tow aspect ratio of fromabout 8 to about 18, and a prepreg openness of no greater than about5.0%.
 39. The prepreg of claim 38 , said average tow aspect ratio isfrom about 12.0 to about 14.5.
 40. The prepreg of claim 38 , whereinsaid prepreg has a prepreg average thickness of from about 0.240 mm toabout 0.300 mm.
 41. The prepreg of claim 38 , wherein said crosssectional shape is a diamond shape.
 42. The prepreg of claim 38 ,wherein said cross sectional shape is a spear shape having two pointedends.
 43. The prepreg of claim 38 , wherein each of said tow strands hasa total fiber filament count of from about 1,000 to about 18,000. 44.The prepreg of claim 38 , wherein each of said tow strands has a totalfiber filament count of about 3,000.
 45. The prepreg of claim 38 ,wherein said fabric has an areal weight of from about 150 to about 250grams per square meter.
 46. The prepreg of claim 38 , wherein saidfabric has a plain weave pattern.
 47. The prepreg of claim 38 , whereinsaid fabric has a satin weave pattern.
 48. The prepreg of claim 38 ,wherein said resin composition comprises a multifunctional epoxy resinhaving an average epoxy functionality of greater than 2.0, and a tan δof from about 0.5 to about 2.0 throughout the elevated temperature rangeof from about 70 to about 140° C.
 49. A core crush resistant prepreg foruse in making a fiber reinforced composite structure, comprising: awoven fabric consisting essentially of carbon fiber tow strandsimpregnated with a hardenable polymeric resin composition comprising anepoxy resin, said tow strands having a predetermined substantiallystable non-round cross sectional shape and a filament count of about3,000; said fabric having an areal weight of from about 180 to about 205grams per square meter; said prepreg having an average tow aspect ratioof from about 12.5 to about 14.0, a prepreg openness of no greater thanabout 3.0%, and a prepreg thickness of from about 0.250 mm to about0.280 mm.
 50. A method for evaluating core crush resistance propertiesof a prepreg for use in a fiber reinforced composite structurecomprising: determining an average tow aspect ratio of the prepreg;determining a prepreg thickness; and comparing said average fiber towaspect ratio and prepreg thickness to a set of predetermined values. 51.The method of claim 50 , wherein said method further comprises:determining a prepreg openness; and comparing said prepreg openness tosaid predetermined values.
 52. The method of claim 51 , wherein saidprepreg comprises a woven fabric consisting essentially a plurality ofcarbon fiber tow strands impregnated with a hardenable resincomposition, said fabric having an areal weight of from about from about150 to about 250 grams per square meter; and wherein said predeterminedvalues are 15.5 and 0.245 mm for average fiber tow aspect ratio andprepreg thickness respectively.
 53. The method of claim 52 , whereineach of said tow strands has a total filament count of from about 1,000to about 12,000.
 54. The method of claim 52 , wherein each of said towstrands has a total filament count of about 3,000.
 55. A method forevaluating core crush resistance properties of a prepreg for use in afiber reinforced composite structure, said prepreg comprising a wovenfabric consisting essentially of carbon fiber tow strands impregnatedwith a hardenable polymeric resin composition comprising an epoxy resin,said fabric having an areal weight of from about 180 to about 205 gramsper square meter, each of said tow strands having a total filament countof about 3,000, said method comprising: determining an average towaspect ratio of the prepreg; determining a prepreg thickness of theprepreg; determining a prepreg openness of the prepreg; and comparingsaid average fiber tow aspect ratio, prepreg thickness, and prepregopenness to a set of predetermined values.
 56. A fiber reinforcedcomposite structure, comprising: a laminate adhered to a lightweightcore, said laminate being formed from at least one core crush resistantprepreg, said prepreg comprising: a woven fabric consisting essentiallyof carbon fiber tow strands impregnated with a hardenable polymericresin composition, said fabric having an areal weight of from about 150to about 400 grams per square meter, said resin composition having anaverage epoxy functionality of greater than 2.0, and a tan δ of betweenabout 1.0 and about 2.0 at 70° C., between about 0.7 and about 2.0 at100° C., and between about 0.5 and about 2.0 at 140° C. or the highesttemperature of minimum resin viscosity; and said prepreg having anaverage fiber tow aspect ratio of less than about 15.5, a prepregthickness of at least about 0.245 mm, and a prepreg openness of fromabout 1.0% to about 10%.
 57. The fiber reinforced composite structure ofclaim 56 , wherein said prepreg has an average fiber tow aspect ratio ofless than about 13.0.
 58. The fiber reinforced composite structure ofclaim 56 , wherein said prepreg has a prepreg thickness of at leastabout 0.260 mm.
 59. A fiber reinforced composite structure, comprising:a laminate adhered to a lightweight core, said laminate being formedfrom at least one core crush resistant prepreg, said prepreg comprising:a woven fabric impregnated with a hardenable polymeric resincomposition, said woven fabric having an areal weight of from about 150to about 400 grams per square meter and consisting essentially of aplurality of carbon fiber tow strands in a first direction interweavingwith a plurality of carbon fiber tow strands in a second direction, saidtow strands in said first direction having a different tow constructionfrom said tow strands in said second direction, wherein said prepreg hasa tow aspect ratio of no greater than about 13.0 in said first directionand a tow aspect ratio of at least 13.5 in said second direction, and aprepreg openness of no greater than 4.0%.
 60. The fiber reinforcedcomposite structure of claim 59 , wherein said tow strands in said firstdirection are twisted tows and said tow strands in said second directionare never twisted.
 61. The fiber reinforced composite structure of claim59 , wherein said resin composition comprises an epoxy resin having anaverage epoxy functionality of greater than 2.0 and a tan δ of fromabout 0.5 to about 2.0 throughout the elevated temperature range of fromabout 70 to about 140° C.
 62. A fiber reinforced composite structure,comprising: a laminate adhered to a lightweight core, said laminatebeing formed from at least a core crush resistant prepreg, wherein saidprepreg comprising: a woven fabric consisting essentially of carbonfiber tow strands impregnated with a hardenable polymeric resincomposition, said tow strands having a predetermined substantiallystable non-round cross sectional shape, said fabric having an arealweight of from about 150 to about 400 grams per square meter, saidprepreg having an average tow aspect ratio of from about 12.0 to about14.5, and a prepreg openness of no greater than about 4.0%.
 63. Theprepreg of claim 62 , wherein said cross sectional shape is a diamondshape.
 64. The prepreg of claim 62 , wherein said cross sectional shapeis a spear shape having two pointed ends.
 65. A core crush resistantprepreg for use in making a fiber reinforced composite structure,comprising: a woven fabric consisting essentially of carbon fiber towstrands impregnated with a hardenable polymeric resin composition; saidfabric having a satin weave pattern and an areal weight of from about150 to about 250 grams per square meter; and said prepreg having anaverage fiber tow aspect ratio of less than about 15.4, a prepregthickness of at least about 0.245 mm, and a prepreg openness of fromabout 1.2% to about 6%.
 66. A core crush resistant prepreg for use inmaking a fiber reinforced composite structure, comprising: a wovenfabric consisting essentially of carbon fiber tow strands impregnatedwith a hardenable polymeric resin composition; said fabric having atwill weave pattern and an areal weight of from about 150 to about 250grams per square meter; and said prepreg having an average fiber towaspect ratio of less than about 15.4, a prepreg thickness of at leastabout 0.245 mm, and a prepreg openness of from about 1.2% to about 6%.